1. Field of the Invention
The present invention relates to a light-emitting device using a light-emitting element that emits phosphorescence or fluorescence. A disclosed embodiment of the present invention relates to a light-emitting device in which light-emitting elements that emits light having different colors are combined. Note that the term light-emitting device encompasses an image display device, a light-emitting device, and a light source (including a lighting device).
2. Description of the Related Art
In recent years, light-emitting devices which have electroluminescence (hereinafter, referred to as EL) elements using EL as self-luminous light-emitting elements have been actively researched. The light-emitting devices are used for organic EL display devices, organic EL lighting devices, and the like. Such light-emitting devices are also called organic light-emitting diodes. The light-emitting devices provided with EL elements have features suitable for display of moving images, such as high response speed and low-voltage, low-power consumption driving, and thus they have attracting great attention as next-generation display devices such as new-generation mobile phones or personal digital assistants (PDA).
For a light-emitting device in which EL elements are arranged in a matrix, a driving method such as passive (simple) matrix driving or active matrix driving can be used. It is considered that an active matrix type structure in which a switch is provided in each pixel (dot) is advantageous to a device having an increased pixel density, since this structure enables driving at a lower voltage. The EL elements are roughly classified into an inorganic EL element using an inorganic compound and an organic EL element using an organic compound (also referred to as an organic light-emitting element).
Further, an organic compound layer has a stacked-layer structure typified by “a hole-transport layer, a light-emitting layer, and an electron-transport layer”. Materials for these layers are roughly classified into low molecular (monomer) materials and high molecular (polymer) materials, and film formation of the low molecular materials is carried out by an evaporation method, for example.
An organic EL element has an anode, a cathode, and the organic compound layer which emits light by application of an electric field. Layers involved in light emission which are included in the stacked-layer structure of the organic compound layer between the anode and the cathode are collectively referred to as an organic EL layer in this specification. As luminescence from an organic compound, light emitted when the compound returns from a singlet excited state to the ground state (fluorescence) and light emitted when the compound returns from a triplet excited state to the ground state (phosphorescence) are known.
Unlike a liquid crystal display device which needs a backlight, an organic EL display device having organic EL elements is a self-luminous device and thus it has excellent visibility due to its capability of easily realizing high contrast and its wide viewing angle. That is, the organic EL display device is more suitable for a display device for outdoor use than a liquid crystal display. A variety of applications of the organic EL display device, such as a display device of a mobile phone or a digital camera, and the like, have been proposed.
Furthermore, with light-emitting elements utilizing electroluminescence, i.e. EL elements, a planar light-emitting device having a large area can be easily formed. This is a feature that is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. In addition, the light-emitting element is estimated to have higher emission efficiency than incandescent lamps or fluorescent lamps. Therefore, the light-emitting element has attracted attention as a preferable next-generation lighting device.
Further, in an organic EL layer, organic compound layers that emit light having different colors can be stacked: for example, layers that emit light having three primary colors, i.e. red, green, and blue can be stacked to form an organic EL light-emitting device that provides white light emission. This can be used, for example, as a backlight of a liquid crystal panel or as a lighting device. However, since emission characteristics of the colors vary over time individually, such a light-emitting device has a disadvantage that the color tone is changed from the initial one. In order to solve this, techniques are developed in which two organic EL elements each including two electrodes having a light-transmitting property are stacked to mix light emission colors and the voltages applied to the EL elements are separately controlled so that a change in color tone is suppressed. For example, conventional techniques described in Patent Documents 1 and 2 are known. These techniques are applicable to modulation of light emission colors, so that one light source can emit light with different color tones. Since the color of light from a lighting device can thus be changed depending on circumstances, a wide variety of applications including an elaborately designed lighting device have been contemplated. Note that, in the case of the light-emitting device in which organic EL elements whose two electrodes have a light-transmitting property are stacked, since light is emitted from opposite sides of the stacked structure forming the light-emitting device, a reflective plate is provided on one side and the light is extracted from the other side. Instead of an electrode having a light-transmitting property, an electrode having a high property of reflecting visible light rays can be provided as the reflective plate on the backmost part of the light-emitting device, so that the reflective plate can also serve as an electrode.